1. Technical Field
The present invention relates in general to wiring board constructions and methods of making them. The invention more particularly relates to methods of making microvias in wiring boards.
2. Background Art
Plated through holes have been employed in printed wiring boards for establishing electrical connections between the top and bottom sides of the boards. Such plated through holes have also been employed in multiple layer wiring board constructions.
According to the construction of conventional plated through holes, relatively large pads surround the plated through holes according to conventional design techniques, because the plated through holes are created by drilling through the conductor pad and the board. The pad is electrically connected to a conductor trace, and the conventional pads are substantially larger in size than the trace to accommodate any potential layer-to-layer or pattern-to-hole misregistration problems during the construction of the plated through hole. As explained in the book, entitled "Printed Circuit Handbook", Fourth Edition, by Clyde F. Coombs, Jr., published by McGraw-Hill, the misregistration is caused principally by the instability and movement of the base laminate substrate during the process of making the printed wiring board or multilayer board.
By having the large pad surrounding the hole, an unwanted and undesirable waste of valuable space on the wiring board results. As stated in the Coombs book, a reduction in the pad diameter from 55 to 25 mils causing a 55% reduction in an area. Such a reduction would double the overall interconnection density. Thus, it is important to reduce the pad diameters. The reduction of the pad diameters would be highly desirable for increasing greatly the wiring capacity of the modern complex printed wiring boards.
Thus, it would be highly desirable to have a new and improved wiring board construction and method of making it to enable through holes to be provided with little or no surrounding pads.
Additionally, the forming of through holes by drilling through the pads individually is time consuming and expensive. Considering the fact that a large number of such through holes are required according to modern manufacturing techniques, it would be highly desirable to enable the individual drilling to be eliminated.
Therefore, the formation of through holes in printed wiring boards and multilayer boards in one operation without the necessity of drilling each one individually would be highly advantageous for cost saving and through put purposes. Also, it would be highly desirable to form the through holes in a highly precise manner to eliminate or greatly reduce the problem of misregistration.